When we illuminate something, we usually expect that the brighter the source we use, the brighter the resulting image will be. This rule also works for ultra-short pulses of laser light – but only up to a certain intensity. The answer to the question why an X-ray diffrac­tion image darkens at very high X-ray intensities does not only deepen fundamental understanding of the light-matter interaction, but also offers a unique perspective for the production of laser pulses that have signi­ficantly shorter pulse duration than those currently available.

When silicon crystals are illu­minated with ultrafast laser pulses of X-ray light, the resulting diffraction images are indeed initially brighter the more photons fall on the sample, i.e., the higher the beam intensity. Recently, however, the counter­intuitive effect has been observed: when the intensity of the X-ray beam starts to exceed a certain critical value, the diffraction images unexpectedly weaken. This puzzling pheno­menon has just been explained, thanks to the efforts of the experimental and theoretical physicists from Japanese, Polish and German research institu­tions, including the RIKEN SPring-8 Centre in Hyogo, the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow and the Center for Free-Electron Laser Science (CFEL) at the DESY laboratory in Hamburg.

X-ray free-electron lasers (XFELs) generate very powerful X-ray pulses with durations of femto­seconds. Machines of this type, currently operating at only a few locations in the world, are used, amongst others, to analyse structure of matter by means of X-ray diffraction. With this technique, a sample is illuminated by an X-ray pulse and the diffracted radiation is recorded. The obtained diffrac­tion image is then used in order to reconstruct the original crystal structure of the material under examination. “Intuition tells us that the more photons we have, the clearer the diffrac­tion image of the sample should be. This is indeed the case, but only up to a certain X-ray intensity, of the order of tens trillions of watts per square centimeter. When this value is exceeded – and we have been only recently capable of doing this – the diffraction signal suddenly starts to weaken. Our research is the first attempt to explain this unexpected effect,” says Beata Ziaja-Motyka, who deals with theo­retical modelling and computer simulations of phenomena related to the interaction of ultrafast X-ray pulses with matter.

Theoretical research undertaken to explain the results of the experiment with XFEL laser firing on crys­talline silicon samples at Japan's XFEL facility has been supported by computer simu­lations. The following explanation of the phenomenon observed emerged from the researchers' work. “When an avalanche of high-energy photons hits a material, there is a massive knockout of electrons from various atomic shells, resulting in a rapid ioni­zation of atoms in the material. Last year, our group showed that the first movements of ionised atoms in the crystal lattice, initiating the process of structural self-destruction of the sample, occured with a delay of approxi­mately 20 femtoseconds after the light pulse hit the sample. We are now convinced that the reason for the recently observed weakening of the diffraction signal is due to phenomena occurring earlier, in the first six femtoseconds of the interaction,” says Ichiro Inoue from the RIKEN SPring-8 Centre.

During the initial phase of X-ray-matter interaction, incoming high-energy photons rapidly excite not only valence electrons from atoms, but also the electrons occupying deep atomic shells, located close to the atomic nucleus. It turns out that the presence of deep shell holes in atoms strongly reduce their atomic scattering factors, i.e., the quantities determining the intensity of the observed diffraction signal. “Our research shows that before any structural damage to the material occurs and the sample dis­integrates, first a rapid electronic damage occurs. As a result, the final part of the pulse practically no longer ionises the material, because further excitation of electrons by X-ray photons is no longer energe­tically possible,” Ziaja-Motyka specifies.

At first glance, the observed effect appears to be just unfavourable, as it results in a decreased brightness of the diffrac­tion images recorded. However, it seems that one can very well exploit this finding. The observation that different atoms respond differently to ultrafast X-ray pulses may help to more accurately reconstruct three-dimensional complex atomic structures from the recorded diffraction images. Another area of potential appli­cation has to do with the production of laser pulses with extremely short pulse durations. Since the material through which the high-intensity X-ray pulse passes cuts off a significant part of the already ultra-short pulse, it can be deli­berately used as scissors to generate pulses that are effectively shorter than those produced so far. If successful, this could stimulate another break­through in imaging of quantum world. (Source: IFJ PAN)

Reference: I. Inoue et al.: Femtosecond Reduction of Atomic Scattering Factors Triggered by Intense X-Ray Pulse, Phys. Rev. Lett. 131, 163201 (2023); DOI: 10.1103/PhysRevLett.131.163201

Link: Institute of Nuclear Physics, Polish Academy of Sciences, Krakow, Poland